The present invention relates to a concave grating spectrometer which disperses and focuses the light to be measured depending on its wavelength and, particularly, to a concave grating spectrometer which provides the spectral image free of astigmatism adapted for use in a polychromator using a semiconductor photo detector.
Among various types of spectrometers which have been employed in polychromators, the most popular one is the concave grating spectrometer which has a very simple optical system and does not need an independent focusing device such as a spherical mirror. The spectral image dispersed and focused depending on its wavelength by the spectrometer is detected by a photo detector which may be a dry plate, or a plurality of photomultiplier tubes or solar cells disposed at the respective focusing positions of the spectrum. Recently, there have been practiced photo detector arrays in which a plurality of small semiconductor photo sensors such as charge coupled devices (CCDs) and photodiodes are aligned at a constant spacing. This photo detector array is formed by integrating several hundreds or more of the above-mentioned semiconductor photo sensors on a line, providing easiness of handling and also allowing the measurement of the light intensity for many wavelengths in a very short time. Accordingly, by combining a photo detector array with the aforementioned concave grating, a polychromator capable of high-speed measurement with a simple structure can be realized.
However, the currently available photo detector arrays are those in which photo sensors are arranged in a flat field and therefore, in order to use the photo detector array, it is a prerequisite for the spectrometer to have an optical system which focuses a spectral image on the same plane. As shown in FIG. 1, the conventional concave grating 1 wherein constantly spaced, straight grooves (on a blank plane) are formed on a concave spherical surface (refer to e.g. HANDBUCH DER PHYSIK, by G. W. Stroke, Vol. XXIX, 1967, pp. 472-486) creates a spectral image surface which is a cylindrical surface including a Rowland circle 2 with a diameter equal to the radius of the spherical surface and normal to the drawing, and the above-mentioned condition is not satisfied. In FIG. 1, reference number 3 denotes an entrance slit, 4 is a beam of sample light to be measured, 5 is the normal of the diffraction grating surface, 6 is a beam of the dispersed monochromatic light, and A is the center of curvature of the spherical surface.
To cope with this problem, there has been employed a method of using a concave grating with such a horizontal focal line which is called lemniscate, wherein grooves are formed in variably spaced, curved configuration by a holographic method using the interference of laser, and the relatively linear portions of the horizontal focal line are used to produce an approximate planar image surface (refer to HEWLETT-PACKARD JOURNAL, by G. W. Hopkins & A. Schwartz, February 1980, p. 17). This method meets the condition of the planar focusing, but has a problem of remaining astigmatism in the spectral image.
Astigmatism does not practically cause a problem when the photo sensitive area of the detector is dimensioned sufficiently large in the direction of height of the spectral image such as the cases of using dry plates or photomultiplier tubes, however, the photo detector array does not have a large enough height in each photo sensor, resulting in the overflow of the light due to astigmatism, and a significant amount of light will be lost. When measuring the spatial distribution of light intensity in the height direction of the entrance slit by using a 2-dimensional photo detector array which is formed by aligning photo sensors also in the height direction, the presence of astigmatism causes the interference, and the accurate measurement cannot be expected.